2GB vs 3GB is talking about the VRAM: Adding VRAM gives you nothing if you already have enough. It works just like system ram; if you already have 16GB, under almost any senario, adding another 16 doesn't do a dang thing. The 2GB on the 670 is more than enough for a 1080p screen.
On top of that, the difference between the 670 and the 660ti is mostly that the 660ti has a neutered memory bus, so it has limited bandwidth, so it can't even use 2GB of VRAM well, much less 3.
Combine that with the fact that the 670 is a fairly faster card, and it wins, hands down.